Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

[enh <enh@google.com>]

On Wed, May 22, 2019 at 12:21 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, May 22, 2019 at 08:30:21AM -0700, enh wrote:
> > On Wed, May 22, 2019 at 3:11 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > On Tue, May 21, 2019 at 05:04:39PM -0700, Kees Cook wrote:
> > > > I just want to make sure I fully understand your concern about this
> > > > being an ABI break, and I work best with examples. The closest situation
> > > > I can see would be:
> > > >
> > > > - some program has no idea about MTE
> > >
> > > Apart from some libraries like libc (and maybe those that handle
> > > specific device ioctls), I think most programs should have no idea about
> > > MTE. I wouldn't expect programmers to have to change their app just
> > > because we have a new feature that colours heap allocations.
>
> Right -- things should Just Work from the application perspective.
>
> > obviously i'm biased as a libc maintainer, but...
> >
> > i don't think it helps to move this to libc --- now you just have an
> > extra dependency where to have a guaranteed working system you need to
> > update your kernel and libc together. (or at least update your libc to
> > understand new ioctls etc _before_ you can update your kernel.)
>
> I think (hope?) we've all agreed that we shouldn't pass this off to
> userspace. At the very least, it reduces the utility of MTE, and at worst
> it complicates userspace when this is clearly a kernel/architecture issue.
>
> >
> > > > - malloc() starts returning MTE-tagged addresses
> > > > - program doesn't break from that change
> > > > - program uses some syscall that is missing untagged_addr() and fails
> > > > - kernel has now broken userspace that used to work
> > >
> > > That's one aspect though probably more of a case of plugging in a new
> > > device (graphics card, network etc.) and the ioctl to the new device
> > > doesn't work.
>
> I think MTE will likely be rather like NX/PXN and SMAP/PAN: there will
> be glitches, and we can disable stuff either via CONFIG or (as is more
> common now) via a kernel commandline with untagged_addr() containing a
> static branch, etc. But I actually don't think we need to go this route
> (see below...)
>
> > > The other is that, assuming we reach a point where the kernel entirely
> > > supports this relaxed ABI, can we guarantee that it won't break in the
> > > future. Let's say some subsequent kernel change (some refactoring)
> > > misses out an untagged_addr(). This renders a previously TBI/MTE-capable
> > > syscall unusable. Can we rely only on testing?
> > >
> > > > The trouble I see with this is that it is largely theoretical and
> > > > requires part of userspace to collude to start using a new CPU feature
> > > > that tickles a bug in the kernel. As I understand the golden rule,
> > > > this is a bug in the kernel (a missed ioctl() or such) to be fixed,
> > > > not a global breaking of some userspace behavior.
> > >
> > > Yes, we should follow the rule that it's a kernel bug but it doesn't
> > > help the user that a newly installed kernel causes user space to no
> > > longer reach a prompt. Hence the proposal of an opt-in via personality
> > > (for MTE we would need an explicit opt-in by the user anyway since the
> > > top byte is no longer ignored but checked against the allocation tag).
> >
> > but realistically would this actually get used in this way? or would
> > any given system either be MTE or non-MTE. in which case a kernel
> > configuration option would seem to make more sense. (because either
> > way, the hypothetical user basically needs to recompile the kernel to
> > get back on their feet. or all of userspace.)
>
> Right: the point is to design things so that we do our best to not break
> userspace that is using the new feature (which I think this series has
> done well). But supporting MTE/TBI is just like supporting PAN: if someone
> refactors a driver and swaps a copy_from_user() to a memcpy(), it's going
> to break under PAN. There will be the same long tail of these bugs like
> any other, but my sense is that they are small and rare. But I agree:
> they're going to be pretty weird bugs to track down. The final result,
> however, will be excellent annotation in the kernel for where userspace
> addresses get used and people make assumptions about them.
>
> The sooner we get the series landed and gain QEMU support (or real
> hardware), the faster we can hammer out these missed corner-cases.
> What's the timeline for either of those things, BTW?
>
> > > > I feel like I'm missing something about this being seen as an ABI
> > > > break. The kernel already fails on userspace addresses that have high
> > > > bits set -- are there things that _depend_ on this failure to operate?
> > >
> > > It's about providing a relaxed ABI which allows non-zero top byte and
> > > breaking it later inadvertently without having something better in place
> > > to analyse the kernel changes.
>
> It sounds like the question is how to switch a process in or out of this
> ABI (but I don't think that's the real issue: I think it's just a matter
> of whether or not a process uses tags at all). Doing it at the prctl()
> level doesn't make sense to me, except maybe to detect MTE support or
> something. ("Should I tag allocations?") And that state is controlled
> by the kernel: the kernel does it or it doesn't.
>
> If a process wants to not tag, that's also up to the allocator where
> it can decide not to ask the kernel, and just not tag. Nothing breaks in
> userspace if a process is NOT tagging and untagged_addr() exists or is
> missing. This, I think, is the core way this doesn't trip over the
> golden rule: an old system image will run fine (because it's not
> tagging). A *new* system may encounter bugs with tagging because it's a
> new feature: this is The Way Of Things. But we don't break old userspace
> because old userspace isn't using tags.
>
> So the agreement appears to be between the kernel and the allocator.
> Kernel says "I support this" or not. Telling the allocator to not tag if
> something breaks sounds like an entirely userspace decision, yes?

sgtm, and the AT_FLAGS suggestion sounds fine for our needs in that regard.

> --
> Kees Cook

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